1. Field of the Invention
The invention relates to a method of pulling the free end of a needle thread from the top to the bottom side of at least one work piece upon a first stitch of a seam that is to be sewn by a sewing machine, comprising a needle which is movable in up and down reciprocation, guiding a needle thread that has been taken from a needle-thread supply by a thread lever; at least one presser to be placed on, and lifted off, the at least one work piece; and a rotarily drivable hook, a tip of which seizes a needle-thread loop and extends it for forming a stitch, with the needle thread being held tight between the needle and the thread lever while the needle-thread loop is extended so that a free end is pulled through the work piece by the hook tip, and with the presser being at least partially relieved while the needle thread is held tight. The invention farther relates to a sewing machine for putting the method into practice.
2. Background Art
A method and a sewing machine of the generic type are known from U.S. Pat. No. 4,658,752. In this sewing machine, the pressure of the work-piece presser drive is adjustable, which is intended to enable even particularly thin, easily snapping threads and especially thick threads to be used, it being further possible to adjust the frictional force, acting on the needle thread, of the thread clamp.
It has been found that as work-piece thickness grows, pressure relief of the presser decreases i.e., when the thickness of the at least one work piece grows, the reliability decreases with which the free end of the thread, namely the starting end of the thread, is pulled from the top to the bottom side of the work piece upon a first stitch.
It is an object of the invention to embody a method of the generic type and a sewing machine for putting the method into practice such that adaptation to various sewing conditions, in particular to varying thickness of the at least one work piece, is attained.
In a method of the generic type, this object is attained in that the duration of relief rises as the thickness of a work piece increases. Preferably, this is done indirectly by the relief time depending on the length of stroke of the at least one presser, input of the length of stroke in adaptation to the sewing job being possible on the side of the machine.
In a sewing machine for putting into practice the method according to the invention, this object is attained in that a presser-relief drive is triggered by the control system in accordance with a function which is recorded in the control system, reflecting the dependency of the time of actuation of the presser-relief drive on the thickness of the at least one work piece. The invention is employable by special advantage in a sewing machine with alternating pressers and needle feed.
Details of the invention will become apparent from the ensuing description of an exemplary embodiment, taken in conjunction with the drawing.